This invention relates to frequency control circuits and more particularly to an electronic system that provides a highly flexible frequency stabilization control.
There is a need for a frequency stabilizer that is simple in construction, economical to manufacture and simple and efficient to use.
Conventional variable frequency oscillators (VFO) have been used for decades for tuning various types of radio and test equipment. However, oscillator drift is often caused by temperature variations that occur because of changes in ambient temperature and self heating of the components. These frequency changes can sometimes be reduced by the use of temperature sensitive components applied in a way to counteract the changes occurring to the oscillator frequency. Such temperature compensation is a tedious process and often is not consistent
In other cases, the oscillator frequency may change because of changes in circuit loading or because of changes in operating voltage, for instance when a unit is powered by battery and the battery is running down.
Frequency synthesizers were developed to correct the frequency drift problem of VFO's and to provide a means of electronic control of oscillators from computer type equipment in addition to manual operator input.
The principle methods of frequency synthesis are the phase locked loop (PLL) and direct digital synthesis (DDS) systems and certain combinations of both. Another method being used by some is the use of a conventional electronic or mechanically tuned oscillator that is stabilized by a digital circuit which locks the oscillator to specific increments of frequency within the operating range of the oscillator.
Phase locked loops are a popular method of creating a stable VFO with digital features. However, if small tuning step size is desired, either circuit complexity increases or performances, with respect to phase noise, decreases. Because the loop is continually correcting phase errors in the VFO signal, noise is added to the output of the oscillator outside the loop bandwidth. Depending upon operating conditions, this added noise can cause a reduction in receiver performance on weak signals in the presence of strong signals. The PLL can reduce noise near the oscillator frequency including microphonics if the loop bandwidth permits. It is also common to encounter spurious responses when tuning across strong signals.
Direct digital synthesizers can produce very small tuning steps with low phase noise, but their upper frequency of operation is limited in practical applications and they add discrete spurious signals up and down the frequency spectrum around the operating frequency. These spurs can usually be suppressed below required levels for transmitter applications but they may not be sufficiently low for use in high performance receiver applications. To reduce these spurious responses, a wide bandwidth PLL operating as a tracking filter is often added to the DDS output, adding complexity, power consumption and cost. A DDS can switch frequency in microseconds if needed and can produce phase shift keyed outputs facilitating applications requiring such features. Like a PLL, the DDS requires a specially programmed controller for each application
The present system combines the best feature of conventional VFO's and digital control circuitry to produce oscillators with high frequency stability and a means to provide electric control in addition to manual operator input.
Unlike a PLL, the present system cannot reduce microphonics or noise near the oscillator frequency nor can it switch the oscillator frequency in microseconds like a DDS. However, it can tune the oscillator in small steps and, like the "huff and pull" system, it does not add noise or spurious signals in the process. It also provides very good long term frequency stability. Because the controller corrects frequency drift only as needed, and since drift in an oscillator is usually in one direction only, the system does not "hunt".
Another feature of the present system is that no special programming of the controller is required to use it. It can literally be moved from application to application without consideration of frequency step size, frequency limits and so on (within its operating range).
The circuit may be compared to a frequency controller chip. The chip measures the frequency of a voltage-controlled oscillator (VCO), reads a shaft encoder, controls the frequency, provides RIT capability, stores frequencies and setup parameters in any of 32 memories and displays measured frequency with a number of available offsets and multipliers. Unlike many designs using phase--locked loops (PLLs) and direct digital synthesis (DDS), techniques, this system does not require use of a controller preprogrammed with specific frequency limits or step sizes for the intended application. It is a general-purpose controller. The circuit provides a 10 Hertz tuning resolution over the frequency range of near DC to 50 MHZ and works at higher frequencies by using prescalers or frequency mixing schemes.
The circuit according to the invention consists of three major sub-systems; namely, a display and control module, an integrator module, and a voltage controlled oscillator. The electronic system disclosed provides a highly flexible method of frequency stabilization and control.
The display and control unit is capable of directly or indirectly measuring the frequency of an electrically tuned oscillator (VCO); reading an operator input device, a control signal input from another controlling device, or from a memory storage system to modify current operating frequency and parameters; controlling the frequency of the VCO by sending control pulses to an integrator or similar circuit; providing incremental tuning capability; storing frequencies and operating parameters in a variable number of memories and optionally displaying measured frequency with offsets and multipliers if desired.
The integrator converts control pulses from the control unit to a DC control voltage/current that is fed to the VCO. The integrator therefore functions as a digital to analog converter. The integrating resistor and capacitor used in the integrator circuit set the time constant of the integrator, which is one factor in establishing the tuning and correction rate of the system. In addition, the integrator may support incremental tuning functions by shifting the output voltage/current of the unit an amount proportional to the incremental tuning control input.
The VCO can be any well designed voltage or current controlled oscillator. It is important to have a VCO with a reasonably linear transfer characteristic. The more linear the transfer function, that is, the ratio of change in control voltage or current to the change in frequency of the oscillator, the faster the control loop can be made to respond over the operator range of the oscillator without risk of instability and the incremental tuning rate will be more consistent at various points within the tuning range of the oscillator.
Applicant is aware of the following U.S. Pat. Nos.: 4,041,416; 4,121,170; 4,453,269; 4,921,467; and, 5,542,113. None of the above patents show a system having the advantages of the present invention.